Many microwave circuits require a voltage-controlled resistor for controlling the amplitude of a received signal. For example, the amplitude of a signal received by a cellular phone depends on the distance from the transmitter. Strong signals need to be attenuated for further processing. Typically, the control signal frequencies are orders of magnitude smaller than signal frequencies. In the cellular phone example, the control voltage needs to respond to changes in signal strength due to movement of a car and thus need only have a response time on the order of milliseconds, while a typical radio-frequency (RF) signal changes amplitude in microseconds for frequencies in the megahertz range and nanoseconds for frequencies in the gigahertz range.
Integrated circuits require a voltage-controlled resistor implemented as a solid state device. One effective device is a P-I-N diode. However, this device requires a high operating current is not ideally suited for hand held consumer devices utilizing small power supplies.
Another device utilized as a voltage-controlled resistor is a metal-semiconductor field-effect transistor (MESFET). FIG. 1 is a diagram of a MESFET connected as a voltage-controlled resistor. Referring to FIG. 1, the MESFET 10 includes source and drain terminals 12 and 14, a channel 16, and metal gate 18. The source and drain metal contacts are ohmically coupled to the source and drain terminals. The metal gate forms a Shottky contact with the channel.
When operating as a voltage-controlled resistor the source and drain terminals function as the RF terminals of the resistor. The gate terminal is connected to control voltage which creates a depletion region 20 under the gate having a depth controlled by the magnitude of the control voltage. As the gate voltage is negatively increased the depletion region moves deeper into the channel thereby reducing the cross-sectional area through which current can flow and increasing the channel resistance.
As is well-known a capacitance is associated with depletion area formed under the gate. A high frequency signal, coupled to the source/drain terminals of the MESFET splits into two components, a main component passing through the channel and the an unwanted component going through the depletion capacitance, the low resistance metal gate, and back into the channel. The unwanted signal component that passes through the depletion capacitance modulates the channel resistance at the signal frequency, resulting in distortion.
A publication entitled The Bootstrapped Gate FET (BGFET)--A New Control Transistor, by Bayruns et al. GaAS IC Symposium, 0-7803-2966-X/95 describes a solution to the problem. A p-type gate is implanted below the channel.
Accordingly, research continues to develop an efficient voltage-controlled resistor in the form of a semiconductor device that can be utilized with integrated circuits.